The present invention relates to cardiac pacemakers, and more specifically to a pacemaker having a selectable hysteresis feature which compensates for sinus node malfunction.
It is well known that natural heart activity, including the depolarization of the sinus node provides optimum hemodynamic performance. Atrial or ventricular stimulations induced by such devices as cardiac pacemakers, generally delay or inhibit natural heart activity by preventing the depolarization of the sinus node.
The hysteresis feature was developed to address this concern, by allowing the pacemaker to follow the sensed sinus node depolarization to a certain predetermined rate below the programmed lower rate of the pacemaker. As such, the escape interval in conventional demand pacemakers equipped with hysteresis feature, is longer than the lower rate interval, for enabling the patient's intrinsic rhythm to control the heart as long as the intrinsic rate is maintained above a predetermined minimum rate. However, in selected patients, these conventional pacemakers do not generally allow the natural heart activity to resume normally after pacing.
The following patents provide a brief historical background for the development and use of the hysteresis feature as it relates to cardiac pacing technology. U.S. Pat. No. 4,856,523, entitled "RATE-RESPONSIVE PACEMAKER WITH AUTOMATIC MODE SWITCHING AND/OR VARIABLE HYSTERESIS RATE," issued to Sholder et al, on Aug. 15, 1989, describes the inclusion of the hysteresis feature in a rate-responsive pacemaker, in an attempt to prevent competition between the pacemaker and the heart's SA node, when the anterograde conduction path is restored. The Sholder patent proposes to vary the hysteresis rate as a function of the pacemaker sensor rate, to a predetermined level upon sensing of the natural heart contraction during the escape interval, as illustrated in FIG. 3B and 4.
U.S. Pat. No. 4,363,325 entitled "MODE ADAPTIVE PACER," issued to Roline et al, on Dec. 14, 1982, and assigned to Medtronic, Inc., discloses a multiple-mode pacer which automatically switches from an atrial synchronous mode to a ventricular inhibited mode when the intrinsic atrial rate drops below a preset hysteresis rate. The Roline patent is incorporated herein by reference.
While the above cited patents and other publications and studies relating to the hysteresis feature have attempted with varying degrees of success to allow the patient's intrinsic rhythm to control, none was completely successful in causing the natural heart activity to resume optimally after pacing.